This application claims priority to provisional U.S. Application Ser. No. 60/276,485, entitled xe2x80x9cFour-Port Wavelength-Selective Crossbar Switches (4WCS) Using Reciprocal WDMs and Optical Circulator Combination,xe2x80x9d invented by Mark D. Feuer et al., filed Mar. 19, 2001, and is incorporated by reference herein. Additionally, the present application is related to provisional U.S. Patent Application Serial No. 60/276,495, entitled xe2x80x9cDelivering Multicast Services On A Wavelength Division Multiplexed Network Using a Configurable Four-Port Wavelength Crossbar Switchxe2x80x9d invented by Mark D. Feuer et al., filed Mar. 19, 2001, and to U.S. Patent Application Serial No. (Atty Docket IDS 2000-502), entitled xe2x80x9cDelivering Multicast Services On A Wavelength Division Multiplexed Network Using a Configurable Four-Port Wavelength Selective Crossbar Switch,xe2x80x9d invented by Mark D. Feuer et al., filed concurrently with the present application, and each of which is incorporated by reference herein.
1. Field of the Invention
The invention relates to wavelength division multiplexed (WDM) signals. More particularly, the present invention relates to a crossbar-type switch for generating an added and dropped wavelength signals having low crosstalk between the dropped and added wavelength signals.
2. Description of the Related Art
FIG. 1 shows a functional block diagram of a conventional four-port wavelength-selective crossbar switch (4WCS) 100. Input wavelengths xcex1, xcex2, . . . , xcexN are demultiplexed first by a wavelength demultiplexer 101, which can be formed by, for example, cascaded thin film filters, fiber Bragg gratings, or arrayed waveguide gratings. The demultiplexed signals are connected through an array of 2xc3x972 crossbar switches 105 to a multiplexer 103 prior to the drop port or an output multiplexer 104. Crossbar switches 105 also connect the wavelengths corresponding to the dropped wavelengths from the add port to output multiplexer 104. The wavelengths that are to be added/dropped are selected by controlling the respective states of the crossbar switches.
A critical problem with a conventional 4WCS, such as shown in FIG. 1, is the potential for optical crosstalk in the 2xc3x972 crossbar switches 105, thereby causing an unwanted portion of the dropped signal to coherently interfere with an added signal. The present invention provides a different implementation of a 4WCS switch and still having the functionality shown in FIG. 1.
Consequently, what is needed is a technique for adding/dropping optical signals from a WDM signal that effectively eliminates optical crosstalk between dropped and added optical signals.
The present invention provides a technique for adding/dropping optical signals from a WDM signal that effectively eliminates optical crosstalk between dropped and added optical signals. The advantages of the present invention are provided by an optical four-port wavelength-selective crossbar switch having a reciprocal wavelength division multiplexer-demultiplexer (WDM MUX-DEMUX) and an optical circulator at the input side, another reciprocal WDM MUX-DEMUX and another optical circulator at the output side, and at least one removable double-sided reflector there between. A WDM demultiplexer separates a wavelength division multiplexed (WDM) signal having a plurality of wavelengths into a plurality of individual wavelength channel signals. A WDM multiplexer combines a plurality of individual wavelength channel signals into a WDM signal having a plurality of wavelengths. Light travels reversibly in a reciprocal WDM MUX-DEMUX. In other words, a reciprocal WDM MUX-DEMUX functions as a WDM multiplexer when individual wavelength channel signals are input from one side and as a demultiplexer when a WDM signal is input from the other side. Each wavelength channel signal corresponds to at least one wavelength of the WDM signal. According to one aspect of the invention, one wavelength channel signal may include a plurality of wavelengths separated by a predetermined free spectral range (FSR) and appears as a comb in the optical spectrum. A reciprocal WDM MUX-DEMUX can be, for example, a waveguide grating router. An optical circulator is a non-reciprocal element including a first port, a second port and a third port. Light input from the first port is coupled to the second port and light input from the second port is coupled into the third port in a circular fashion. The input circulator receives the WDM signal through the first port and couples the WDM signal to the input end reciprocal WDM MUX-DEMUX through the second port. A drop signal that is received through the second port of the input optical circulator is output from the third port (drop port) of the input optical circulator. The output optical circulator receives the output WDM signal from the output end reciprocal WDM MUX-DEMUX through the second port and outputs the output WDM signal through the third port. An add signal that is coupled to the first port (add port) is output from the second port of the output optical circulator to the output end reciprocal WDM MUX-DEMUX. Each double-sided reflector is disposed in a path of a selected wavelength channel signal between the optical demultiplexer and the optical multiplexer, and is selectably operated so that in a first mode of operation a first side of the double-sided reflector reflects a selected wavelength channel signal corresponding to the wavelength channel signal path in which the double-sided reflector is disposed back to the second port of the input optical circulator. A second side of the doubled-sided reflector in the first mode of operation reflects an add signal having at least one wavelength corresponding to the wavelength channel signal path in which the double-sided reflector is disposed back to the second port of the output optical circulator. The selected reflected wavelength channel signal can be modulated with, for example, multicast data (as described in the provisional U.S. Patent Application Serial No. 60/276,495, entitled xe2x80x9cDelivering Multicast Services on a Wavelength Division Multiplexed Network Using a Configurable Four-Port Wavelength Crossbar Switch), and coupled to the add port of the output optical circulator. In a second mode of operation, each double-sided reflector allows the selected wavelength channel signal corresponding to the wavelength channel signal path in which the double-sided reflector is disposed to pass from the optical demultiplexer to the optical multiplexer. In one embodiment of the present invention, at least one double-sided reflector is a micro-electro-mechanical-system (MEMS) mirror. In an alternative embodiment of the invention, the double-sided reflector is a mechanical anti-reflection switch (MARS). In yet another alternative embodiment, the double-sided reflector is a reflective thin-film interference filter. In a further embodiment, a series of reflective thin-film interference filters corresponding to different FSRs are used in place of the double-sided reflective mirrors. This embodiment allows wavelengths corresponding to different FSRs in each wavelength channel signal to be independently set to the bar (through) or cross (add/drop) state.
The present invention also reduces the number of WDM MUX-DEMUXs required to achieve the same function by a factor of two compared with the conventional approach.